This project will examine the relationships between methemoglobinemia, diarrheal disease and nitrate-nitrogen exposure to further illuminate the connections between environmental exposures to nitrate/nitrite through ingestion of potable water containing these substances. A pilot, retrospective cohort study is proposed with the following aims: 1) To confirm the current nitrate/nitrite level in the participants' potable water supply; 2) To examine on the basis of varying levels of Nitrate/Nitrite exposure, individuals > 1 year of age and < 60 years in age-matched cohorts of high (>5 ppm ), medium (1-5 ppm), and low (# 0.9 ppm) exposure categories, the relationship between these varying levels of nitrate/nitrite exposure and: overall health status (including diarrheal disease experiences), methemoglobinemia history and/or self reported symptoms, if any, current methemoglobin level, and lymphocyte response status as an indicator of immuno-competency; (a number of measures including proliferation and cytokine expression); 3) To test the null hypothesis that there are no significant differences between high (>5 ppm), medium (1-5 ppm), and low (# 0.9 ppm) exposure cohorts consuming nitrate containing waters in regard to: overall health status (including diarrheal disease experiences), methemoglobinemia history and/or self reported symptoms, if any, current methemoglobin level, and lymphocyte response status as an indicator of immunocompetency (a number of measures including proliferation and cytokine expression); and 4) To provide further information to inform the scientific debate regarding the relationship between consumption of nitrate/nitrite containing waters and the overall health effects as they may reflect on the current maximum contaminant level, MCL of 10 ppm (mg/I) nitrate/nitrogen, a level that was set without consideration of a safety factor [unreadable] [unreadable]